In high-speed digital data communications, a transmitted digital signal is attenuated by the transmission channel (i.e, transmission medium) through which the digital signal travels. The attenuation causes intersymbol interference (ISI) and increases jitter.
ISI arises because of temporal “spreading” of a transmitted symbol pulse due to a dispersive nature of the channel, which results in an overlap of adjacent symbol pulses. In other words, ISI occurs when a portion of a digital signal representative of one piece of information interferes with a different portion of the digital signal representative of a different piece of information. The transmitted symbol pulses may arrive at a receiver at different times. Components of adjacent symbol pulses may interfere constructively or destructively. Adverse effects of ISI are pronounced when a signal to noise ratio is high and the channel is relatively noise-free. In relatively noise-free channels, the presence of ISI greatly degrades performance of the communication system.
Timing jitter is a temporal disorder related to variability in a latency time of the transmitted digital signal. In other words, the transmission medium through which the digital signal travels introduces a variable phase delay into the transmitted digital signal.
An eye diagram provides a convenient way to evaluate the impact of ISI and jitter-related impairments. The eye diagram is formed by superimposing waveforms of multiple pulse sequences of the transmitted digital signal. The eye diagram typically looks like an eye between a pair of rails. An eye diagram is useful to a circuit designer, because the eye diagram indicates levels of ISI and jitter imparted by the transmission medium into the transmitted digital signal. An open eye pattern indicates minimal signal distortion. A closed eye pattern indicates levels of ISI and jitter so severe that a received signal may be unintelligible.
Once a level of ISI and jitter is determined, full-bit pre-equalization can be used to minimize the effects of ISI. Full-bit pre-equalization in a transmitter boosts the amplitude of the digital signal prior to transmission, so that the digital signal may be more easily decoded at the receiver. The digital signal is altered at the transmitter so that the influence of the channel on the digital signal yields a received digital signal capable of being decoded at the receiver. Conventionally, full-bit pre-equalization increases the amplitude of the digital signal at every digital data transition, and leaves the amplitude unchanged when there is no transition of the digital data.
FIG. 1 illustrates conventional full-bit pre-equalization 100. An input digital data signal 105 (Data_in) to be transmitted is shown. A full-bit pre-emphasis signal 110 (Pre-emph) is also illustrated. When the full-bit pre-emphasis signal 110 is high, full-bit pre-emphasis is enabled, and the amplitude of the state of the input digital data signal 105 is increased for the full duration of each pre-emphasized bit, as shown in the full-bit emphasis data out waveform 115 (Data_out).
An effect of full-bit pre-emphasis is increased noise of the transmitted digital data signal, particularly when multi-level data is transmitted. The added noise decreases a sampling margin at the receiver. For example, in a 4-PAM (4-level pulse-amplitude modulation) output driver, the conventional full-bit pre-emphasis method introduces noise to each of the four logic levels, which leads to data sampling errors at the receiver.
Accordingly, there are long-felt industry needs for apparatus and methods to improve transmission channel efficiency, and improve the eye opening of a transmitted pulse amplitude modulated signal, without affecting the sampling margin.